It has been known for some time that certain parts of the human body, for example the brain and the heart, generate, or respond to, identifiable electrical signals. Furthermore, nerve impulses in general are known to have electrical components that can be detected using appropriate medical electrodes. The study of these signals can be useful in the diagnosis or monitoring of certain conditions and diseases. Generally, these signals are of relatively low strength, and associated with relatively high levels of noise. Consequently, the medical electrodes used to sense them must have relatively high levels of sensitivity. This usually requires that the electrodes be placed directly on the skin so as to make very good electrical contact with the skin surface, and be resistant to the influence of external forces such as those resulting from patient movement.
In many diagnostic or monitoring procedures, the electrodes are worn by the patient for relatively long periods of time. Thus, in addition to being sensitive, the electrodes must also be reasonably comfortable to wear. They cannot cause undue skin irritation, which can affect the results of the monitoring or cause patient discomfort.
It has become commonplace now to use disposable medical electrodes, both for sanitary reasons and to reduce the time and labor that is normally involved in cleaning nondisposable electrodes that may have been used previously. With disposable electrodes, it is important to minimize the cost of the electrodes and still provide the required sensitivity and comfort.
Typical prior art disposable medical electrodes include three basic components: a support layer of compliant material having an adhesive undersurface to permit attachment to the skin; an electrical terminal for attachment to an electrical lead that carries the sensed signals to the monitoring or diagnostic equipment; and, a pad filled with a conductive gel that contacts the skin and provides a good electrical connection between the skin and the terminal. Various techniques have been used to combine these three electrode components into a single unit. For example, in the electrodes disclosed in U.S. Pat. Nos. 3,828,766; 3,901,218; 3,989,035; 4,019,500; 4,063,352; and 4,067,322, the electrical terminal is composed of two parts, typically an eyelet and mating stud, which are pressfit together to form the terminal. The adhesive support layer, or another layer attached to the support layer, is positioned between the stud and eyelet so that when the stud is pressed into the eyelet, the layer is caught therebetween and held in place.
The primary problem with such two-part terminals is their cost. Since both parts of the terminal must be electrically conductive, they both must be made of, or at least coated or plated with, a non-corrosive, electrically conductive material such as silver or silver chloride. Because of this, the electrically conductive terminal parts are generally the most expensive parts of the electrode. Further, with electrodes of this type, the third component, the gel pad, is either not secured to the support layer or terminal, giving rise to motion artifacts in the output signal, or is secured through the use of welding or an adhesive, adding to the cost of manufacturing the electrode.
To avoid the high cost of two-part terminals, various disposable medical electrodes have been proposed that utilize a single-part terminal. Such single-part-terminal electrodes can be divided into two general types. One type, illustrated in U.S. Pat. Nos. 3,868,946; 4,029,086; and 4,066,078, retain the support layer, terminal and gel pad relative to one another by means of adhesive which, as noted above, adds to the cost of manufacturing the electrodes. Electrodes of the other type shown in U.S. Pat. Nos. 3,752,151; 3,865,099; and 3,882,853, utilize rather complicated single piece holding members. In these constructions, the terminal, the support layer or both are actually welded to the holder adding to the cost. Also, the gel pad is either left unsecured, or secured through the use of an adhesive, giving rise to the same disadvantages discussed above.